1. Field of the Invention
The present invention relates to an image forming apparatus and a method of correcting a color registration error.
2. Description of the Related Art
Recently, for electrophotographic color image forming apparatuses, tandem color image forming apparatuses are mainly used, in which a plurality of image forming units are connected in series to form a full-color image. In the tandem image forming apparatuses, images formed by the plurality of image forming units corresponding to colors of yellow, cyan, magenta, black or the like, for example, are primarily transferred on an intermediate transfer belt in an overlapped manner. Then, the overlapped image is secondarily transferred on a printing paper, for example. Further, by fixing the secondarily transferred image on the printing paper, a full-color image is formed.
By the above described tandem image forming apparatuses, productivity (a number capable of being printed per hour, for example) is drastically increased. However, there may be misregistration of images of the plurality of colors due to inaccuracy of positions or differences in diameter of photosensitive drums, exposure equipment or the like, inaccuracy of optical systems, or the like, of the image forming units of the plurality of colors. This misregistration of images of the plurality of colors causes a color registration error in the image formed on the printing paper. Thus, correction of the color registration error is essential.
As the correction of the color registration error, a method is known in which test patterns for detecting color registration errors of the plurality of colors are formed on the intermediate transfer belt, positions of the test patterns are detected by a sensor or the like to calculate color registration error amounts, and, when forming a normal image, optical paths of the respective optical systems are corrected or starting positions to form the respective images or pixel clock frequencies are corrected based on the color registration error amounts.
However, in order to correct the optical path of the optical system, it is necessary to mechanically operate a light source, a correction optical system including an f-θ lens, a mirror in the optical path or the like for each color. Thus, it is necessary to provide a super precision movable member for each color to lead high-cost. Further, as it takes a large amount of time to complete such a correction, it is not easy to perform the correction operation often.
Conventionally, a method is known in which a coordinate position of image data of each color is automatically corrected in order to cancel misregistration by a coordinate converting unit based on a calculated color registration error amount of the respective color, for example (see Patent Document 1, for example). However, this method cannot correspond to a change of the color registration error amount over time due to a deformation of the optical system, the holding member or the like caused by a charge over time or a temperature change or the like in the image forming apparatus.
Thus, a method is known in which the correction of the color registration error is repeated in accordance with a predetermined temperature change or time (see Patent Document 2, for example).
Further, a method is known in which a test pattern is periodically at a position not overlapping with an area where a normal image is formed, and the color registration error amount is updated so that a correction can be performed based on a color registration error amount that corresponds to a temperature change or the like in the image forming apparatus (see Patent Document 3, for example).
Further, a color registration error factor includes a linear component that has linear characteristics regarding a distance in a main-scanning direction or in a sub-scanning direction, and a nonlinear component other than the linear component. The nonlinear component includes a color registration error of the nonlinear component, which is a so-called “bow of a scanning line of a main-scanning”. A method is known in which a correction of high accuracy can be performed by using correction data of the nonlinear component (see Patent Document 4, for example).
However, according to the methods described in the Patent Document 1 and Patent Document 2, the color registration errors are not corrected in real time. Further, when images are printed on both surfaces of a printing paper, an image is printed on a back surface after printing an image on a front surface and passing the printing paper through a fixing device. At this time, water included in the printing paper is evaporated and the printing paper becomes a contracted state when passing through the fixing device. Thus, if the image is printed on the back surface when the printing paper is in the contracted state, there may cause misregistration or distortion in the image formed on the back surface when the size of the printing paper returns to its original size. However, the above described documents do not disclose measures for these problems.
Further, there may be a problem that an image is formed on a printing paper in inclined states when the printing paper is inclined with respect to a conveying direction of the printing paper. However, the above described documents do not disclose measures for this problem.